[EVO 4G Q&A, Help & Troubleshooting] psych2l:
I'm actually going to post a serious reply because I thought it was pretty interesting. The evo is actually employing your hand to increase its signal, so there is no weak spot, only win, as explained by Earlymon from android forums:

If the number and its root mechanisms are that important to you and since you seem to be able to follow engineering research, it goes like this -

1. The Evo contains the following radios: FM, GPS, wifi, WiMAX (4G) and CDMA (3G).

2. Whereas all iPhones have fewer radios, and they're differently configured.

3. An iPhone 4 is susceptible to the moisture in your hand contributing to a tendency for natural conductance that can cross-connect antennas and cause signal attenuation, or act as a shield - everyone's familiar with that concept trying to get reception on any radio during the rain.

4. The Evo's larger size forces you to always wrap your hand around it, you don't have the choice of curling 3 fingers below it and grasping in with index finger and thumb like you can will other phones.

5. This larger-object grip causes your hand to have a natural curl.

6. If you recall, iron filings on a paper on top of a magnet prove that all magnetic fields have a natural tendency to curl.

7. Radio waves are electromagnetic waves.

8. In addition to moisture, your hand is a terminus for blood flow - in other words, given that it's an end-point on your body, blood flows there, curls around and flows out.

9. Blood contains iron.

10. The effect has been understood for decades, but now that HTC exploited all of this first, others will be following suit.

11. The iron in your naturally curled hand is creating a resonance-circuit closure path between the various radio antennas in the Evo.

12. The electromagnetic flow was going to happen anyway, but rather than being chaotic, it's now being preferentially focused for lateral curl and the flowing iron in your hand is significantly reducing the magnetic reluctance for the antenna.

13. The published estimates call for a 0.278% efficiency increase in the antenna when you hold it.

14. Because dB is really meaningless - it's a relative measure! - how about we use the referenced relative measure common to all radio engineers - the dBm, shall we? That was your first clue that Gizmodo was just being sensational.

15. Anyway, depending on various factors such as the size of your hand, your overall health (been getting your iron lately?) - rather than attenuate the signal, that 0.278% efficiency increase is going to result in additional antenna gain of roughly anywhere from an additional 0.9 to 1.738 dBm.

16. Other phones can't do that - that's part of what makes your Evo so special.

For engineers and hard core tech types only:

From the Radio Systems Interface Design Digest, How to Profit from The Second Reference to North in Curled EM Low-Reluctance Circuit Designshttp://rsidd.online.fr/profit/profit.html (online archive, org. pub. 1954, XXIVb34, France)

PS - Outside of iPhone: metal, metal conducts and screws up magnets; outside of Evo: plastic, plastic does not conduct and magnetic waves pass right through it.

There was a reason the original telephones were all plastic ;)